This invention relates to a circuit board, a connector assembly and a cable harness.
Referring to FIG. 7, JP-A 2004-14800 (Patent Document 1) discloses a circuit board 900 on which a pair of differential traces 920 is formed. The differential traces 920 have a pair of first trace portions 921, a pair of second trace portions 928 and a coupling portion 930. The first trace portions 921, the coupling portion 930 and the second trace portions 928 are coupled in this order. The coupling portion 930 has a pair of first coupling portions 932, a pair of third trace portions 933, a pair of through holes 934, a pair of fourth trace portions 936 and a pair of second coupling portions 937. The coupling portion 930 is configured that the first coupling portions 932, the third trace portions 933, the through holes 934, the fourth trace portions 936 and the second coupling portions 937 are coupled in this order. In a Y-direction, the first trace portions 921 are spaced apart from each other by a first distance while the second trace portions 928 are spaced apart from each other by a second distance. If, in the Y-direction, the third trace portions 933 are arranged apart from each other by a distance equal to the first distance while the fourth trace portions 936 are arranged apart from each other by a distance equal to the second distance, the existence of the through hole 934 reduces a differential impedance of a line from the third trace portion 933 to the fourth trace portion 936 as compared with any of a differential impedance of the first trace portion 921 and a differential impedance of the second trace portion 928. This reduction causes impedance mismatch between the coupling portion 930 and each of the pair of the first trace portions 921 and the pair of the second trace portions 928. On the contrary, the differential traces 920 of Patent Document 1 are provided with the first coupling portions 932 and the second coupling portions 937 so that the third trace portions 933 are arranged apart from each other by a distance greater than the first distance while the fourth trace portions 936 are arranged apart from each other by a distance greater than the second distance. This arrangement increases the differential impedance of the line from the third trace portion 933 to the fourth trace portion 936 to prevent impedance mismatch between the coupling portion 930 and each of the pair of the first trace portions 921 and the pair of the second trace portions 928. Additionally, if the differential impedance of the line from the third trace portion 933 to the fourth trace portion 936 is greater than expected, the differential impedance can be adjustably reduced by arranging a conductive member 950, as shown in FIG. 7, between the lines each consisting of the third trace portion 933, the through hole 934 and the fourth trace portion 936.
There is a demand for a cable harness, which comprises a circuit board provided with a pair of differential traces, to enable a mating direction, in which the cable harness is mateable with a mating connector, to be perpendicular to an extending direction in which a cable mainly extends. If a cable, which is composed of a plurality of single wires, is bent so that the extending direction is forced to be perpendicular to the mating direction, the cable largely protrudes outside a connector assembly so that the cable harness as a whole has an increased size. Accordingly, the cable harness having this structure is unfavorable.
One possible approach for arranging the mating direction to be perpendicular to the extending direction without the cable harness having an increased size is to bend the differential traces, which are formed on the circuit board, to L-shape. However, if the differential traces on the circuit board are modified to be simply bent to L-shape to be arranged parallel to each other, an inner trace of the modified differential traces is shorter than an outer trace of the modified differential traces. Accordingly, skew occurs between a differential signal pair which passes through the modified differential traces, and thereby a transmission error occurs. One possible approach for reducing occurrence of the skew is to remodel a part of a shape of the inner trace which is positioned at a bending region of the modified differential traces so that the inner trace and the outer trace have the same length.
If the inner trace is remodeled as described above, no skew occurs between a differential signal pair which passes through the remodeled differential traces. However, the remodeled shape of the inner trace distorts differential signals to cause unnecessary mode conversion where some of differential-mode signals are converted to common-mode signals. Specifically, the remodeled differential traces have a drawback to cause differential to common-mode conversion.